Practical: food tests

Structure and functions in living organisms · Biological molecules and enzymes · note 2 of 5

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Practical: food testsSpec 2.9

In short

Food tests use chemical reagents to identify biological molecules in a food. Benedict's solution, heated in a water bath, turns from blue to brick-red with glucose. Iodine solution turns from orange-brown to blue-black with starch. Biuret reagent turns from blue to purple with protein. Fat shaken with ethanol and poured into water forms a cloudy white emulsion.

You can use chemical reagents to identify the main biological molecules in a food. If the food is a solid, crush it with a little water and use the liquid. For the fat test, shake the food with ethanol instead. Always wear eye protection.

Practical:

A negative result means the reagent keeps its original colour. Learn the start colour as well as the positive result.

Food tests
Food	Reagent	Method	Positive result
Glucose (a reducing sugar)	Benedict's solution	Add Benedict's solution and heat in a hot water bath for a few minutes	Blue to green, yellow, orange or brick-red, depending on the amount of sugar
Starch	Iodine solution	Add a few drops of iodine solution to the sample	Orange-brown to blue-black
Protein	Biuret reagent	Add biuret reagent to the sample and mix	Blue to purple (lilac)
Fat	Ethanol	Shake the sample with ethanol, then pour the mixture into water	A cloudy white emulsion forms

In a negative result, iodine stays orange-brown, Benedict's solution and biuret reagent stay blue, and the ethanol test stays clear. Ethanol is flammable, so keep it away from naked flames. Benedict's solution is heated in a water bath, not directly over a flame.

Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026

Frequently asked questions

How do enzymes speed up reactions?

Enzymes speed up reactions by acting as biological catalysts. The substrate fits into the enzyme's active site because their shapes are complementary, forming an enzyme-substrate complex. The reaction takes place at the active site, then the products leave and the enzyme is free to be used again, because it is not used up.

Why do enzymes denature at high temperatures?

Enzymes denature at high temperatures because the heat damages the enzyme's structure, so the shape of the active site changes. The substrate is no longer complementary to the active site and cannot fit, so the rate of reaction falls quickly towards zero. Denaturing is permanent, and enzymes are not alive, so they are not 'killed'.

What happens to enzymes at low temperatures?

At low temperatures enzymes work slowly but are not denatured. The enzyme and substrate particles have less kinetic energy and move more slowly, so they collide less often and fewer enzyme-substrate complexes form. As the temperature rises towards the optimum, around 37 °C for many human enzymes, collisions increase and the rate rises.

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